This invention relates to a multi-cavity klystron, more particularly a high power klystron amplifier operating in a millimeter wave band.
Multi-cavity klystrons are widely used in high power sources of a microwave band and a millimeter wave band as ground station output tubes for overhorizon communication and satelite communication. In the field of the satelite communication it is a recent trend to expand the operating frequency from prior art 6 GH.sub.Z band to a millimeter wave band. Generally speaking, the multi-cavity klystron can operate at a lower direct current beam voltage than a cavity coupled type travelling wave tube having the same output power thereby miniaturizing the transmitter. For this reason, it is expected that the field of application of the multi-cavity klystron would be expanded and such klystron is most suitable for use as the output tube of a movable station for satelite communication.
However, as the operating frequency increases the dimension of the cavity decreases, and in the millimeter wave band the maximum diameter of the cavity decreases below 10 mm and the diameter of a drift tube through which an electron beam passes also decreases below 1 mm. Accordingly, it has been difficult to obtain a high power klystron amplifier having a large gain.multidot.bandwidth product suitable for communication for the following reasons.
1. As the electron beam diameter is smaller than 1 mm, the electron beam current density becomes high thereby requiring a large focusing flux density. However, the flux density of the presently available focusing device for electronic tubes is less than 8,000 gauses so that the beam perveance is limited to less than 1.times.10.sup.-6 A/V.sup.3/2.
2. In addition to the fact that the electron beam diameter is reduced and the practical value in beam perveance is limited to a small value, the magnitude of the available DC beam current I.sub.0 also tends to decrease with the operating frequency because the cathode diameter of a conventional electron gun is of the order of 10 to 15 times of the electron beam diameter and because the emission current density per unit area of the cathode surface is limited by the useful life of the tube.
3. Since the available DC current beam current I.sub.0 is limited, it is necessary to increase the beam voltage V.sub.0 for the purpose of obtaining a definite output power. Then, the electron beam DC conductance G.sub.0 defined by the following equation decreases. EQU G.sub.0 =(I.sub.0 /V.sub.0)(.upsilon.) (1)
4. as the resistance loss in the cavity wall due to skin effect increases with the operating frequency, in the millimeter band, the unloaded Q of the cavity decreases to about 1300.
5. Since the dimension of the cavity has been decreased and moreover since the power consumption due to the resistance loss of the cavity wall has been increased as above described the amount of heat generation per unit area increases, whereby the resonance frequency is caused to drift due to the thermal expansion of the cavity, thus varying the output of the tube.
6. When the cavity is constructed to be thermally stable for the purpose of preventing the thermal drift of the resonance frequency described above, the characteristic impedance R/Q which is an important electrical parameter for determining the gain.multidot.bandwidth product of the klystron amplifier would decrease below 100 ohms.
In one example of the design of a klystron amplifier operating at a frequency of less than 10 GH.sub.Z, since there is no such basic difficulties, it is possible to increase the electron beam DC conductance G.sub.0 and the characteristic impedance R/Q to the output cavity, it was found that there is the following relationship between these parameters. EQU 1.ltoreq.G.sub.0 (R/Q)Q.sub.ex .ltoreq.2 (2)
Where Q.sub.ex represents an external Q determined by the size of coupling means between the output cavity of the klystron and an external circuit.
The relationship of equation 2 is determined by conditions necessary to increase the gain and the band width of the tube, and to make equal the high frequency voltage generated across the interaction gap of the output cavity to the direct current beam voltage at the saturation output, thereby increasing the saturation output power.
One example of the prior art klystron amplifier operating in the millimeter wave band is a klystron amplifier having a band center frequency of 35 GH.sub.Z and described in B. G. James and L. T. Zitelli paper of the title "Kilowatt CW Klystron Amplifiers at K.sub.u and K.sub.a Bands," The Microwave Journal, 1968, November page 53. In this klystron amplifier, the parameters are selected such that: the DC beam voltage V.sub.0 =10,750 V, the DC beam current I.sub.0 =1.0 A, the unloaded Q of the output cavity =1400, the characteristic impedance of the output cavity R/Q=50, and the external Q of the output cavity Q.sub.ex =350. More particularly, the design parameters of this klystron amplifier have been selected such that the decreases in the electron beam DC conductance G.sub.0 and in the characteristic impedance R/Q of the output cavity are compensated for by selecting a large value 350 for the external Q.sub.ex of the output cavity and that the product G.sub.0 (R/Q).multidot.Q.sub. ex will be 1.63 which is in the range defined by equation 2.
However, in a klystron amplifier operating in the millimeter wave band, if the value of Q.sub.ex were increased so as to satisfy the relationship of equation 2, although the peak value of the output power would increase to some extent, the gain.multidot.bandwidth product would decrease. This is caused by the decrease in the conductance Q.sub.0 of the output cavity and since the circuit efficiency .eta..sub.c defined by the following equation decreases with the increase in the external Q.sub.ex, thus making it difficult to increase the output power as expected. EQU .eta..sub.c =Q.sub.0 /Q.sub.ex +Q.sub.0) (3)
decrease in the circuit efficiency not only limits the power output but also increases the heat generation cuased by the resistance loss in the output cavity thereby causing unstable the operation of the klystron amplifier which is a fatal defect for a practical tube.